A Linear Variable Phase Transformer With Constant Magnitude Output is disclosed in U.S. Pat. No. 4,282,485, granted to Pauwels et al, and assigned to the Pneumo Corporation. Pauwels discloses a bobbin upon which two primaries are mounted which are specially wound as different size donuts and apparently wound in-place according to sine and cosine functions. The bobbin is a long hollow cylinder for receiving a moveable magnetic core which is attached to an actuator rod the displacement of which is to be measured. The series of varying diameter donut shaped windings are wound onto the bobbin and are connected together. The first primary consists of a series of such donut shaped windings, all having the same inside diameter but having outside diameters which vary according to a cosine function. In other words, the diameters of the donuts along the length of the bobbin and transverse thereto will vary according to a cosine function. The first primary winding donuts are connected up in series in the finished unit. A second series of donuts is wound on top of the first set and constitutes the second winding. Both the internal and external diameters of this second series of donuts will vary since the inside diameter of each of the second series of donuts is wound on the outside diameter of a corresponding donut from the first primary. The diameters of the second set of donuts vary according to a sine function. The donuts of the first series are connected in series electrically, as are the donuts of the second series. A secondary winding of uniform thickness is wound on the very first layer of the bobbin underneath the donuts. The primaries are excited by separate sine and cosine signals in quadrature. The core combines the primary signals and induces an output signal in the secondary according the core position relative to the primaries. The signal has a constant magnitude and varies its phase with respect to one of the excitation signals used as a reference.
Thus, Pauwels discloses an LVPT including two segmental and generally coaxial primaries, each segment including a plurality of circumferential turns around the axis. The number of turns in each primary segment and the direction in which they are wound are determined in accordance with sine and cosine functions. Although the discrete nature of the windings makes the transducer somewhat difficult to fabricate and the resulting output signal has certain discontinuities and inaccuracies, there will be a high degree of linearity between the output signal phase and the core displacement. Nevertheless, it would be desirable to avoid some of the manufacturing difficulties due to the work involved in winding a large number of coil steps, including the application of the necessary insulation between the coils and also the overall wire management problem.
While differential transformers having multiple primaries with a single, uniform secondary have been utilized in many applications, it is sometimes necessary to transduce rectilinear motion in other ways. One such requirement is to convert rectilinear motion into multiple electrical signals with magnitudes which vary as trigonometric functions utilizing a single phase AC signal excitation source. The magnitude of the electric signal outputs of such a device vary in the form of sine or cosine functions while the input motion to be measured remains rectilinear motion. A resolver is an apparatus which has previously been used to provide a trigonometric electrical signal with respect to input displacement. A resolver utilizes a shaft which rotates to create the output sine or cosine electrical signal. The input measurement must be in the form of rotational or angular displacement. In order to measure rectilinear motion with a resolver, the rectilinear motion must be converted to angular displacement. Under these circumstances, mechanical gear devices have been utilized to convert rectilinear motion to angular motion. Unfortunately, the gear devices and the resolvers introduce error in the accuracy of the measurement. Also, they are unreliably, bulky, and heavy. A cost effective replacement for resolvers and their accompanying mechanics is, therefore, desirable.